I am sorry you had a frightening time. I wouldn't be surprised if you are undermedicated and hope the GP does the Full Thyroid Function Test. I also read this link that some people are sensitive to the fillers/binders in levo:-
Hypersensitivity reactions to inactive ingredients (in this product or other levothyroxine products) have occurred in patients treated with thyroid hormone products. These include urticaria, pruritus, skin rash, flushing, angioedema, various GI symptoms (abdominal pain, nausea, vomiting and diarrhea), fever, arthralgia, serum sickness and wheezing. Hypersensitivity to levothyroxine itself is not known to occur.
When on levo I also had severe palpitations and the heart investigations proved I had no problems. Since stopping levo I have had no more palpitations or visits to A&E.
If hypothyroid we usually have low acid rather than high and I wonder if lanzoporole which, I understand, is for high acid problems will help. This is an extract re low acid:-
Most people with thyroid conditions and adrenal fatigue will have low stomach acid (hydrochloric acid or HCl), which is necessary to break down protein. This is known as “achlorhydria.” The lack of adequate digestive enzymes leads to a depletion of amino acids, iron, zinc and other nutrients obtained from protein. Symptoms include gas, heartburn, bloating, and heaviness in the stomach after eating a protein meal.
Achlorhydria, constipation, and incomplete digestion of fibrous plant materials have been associated with hypothyroidism.
Liver function tests may be disturbed in up to 50% of people with hypothyroidism, leading to reduced output of bile, which helps us digest lipids. Notably, bile stones and gallstones are also more common in Hashimoto’s.
When the GP prescribed medication for my painful stomach, it was worse and I then learned about low acid and since have taken betaine/pepsin 3 times a day and have had no problems since.
I do hope they can sort out your problems promptly.